FIELD OF THE INVENTION
The present invention relates to a method for the regulation of an asynchronous motor by vector control in an imaginary space vector coordinate system.
Today, many electric motor drives use a frequency controlled a.c. motor. With frequency control, the motor efficiency remains high over the whole speed range and the mains power ratio is almost 1. Moreover, the motor used may be a simple squirrel-cage motor.
In the regulation of an asynchronous motor, equivalent circuits representing the motor may be used. For example, an equivalent circuit applicable in asynchronous motor regulation systems is found in figures 2.20 d) and 2.21 in Buhler's book "Einfuhrung in die Theorie geregelter Drehstromantriebe, Band 1: Grundlagen", 1977, which presents a uniaxial, complex equivalent circuit for the stator and rotor of an asynchronous motor in a coordinate system rotating at the angular velocity of the stator. Also, the same book presents the voltage and flux equations for the asynchronous motor in the coordinate system in question (pp 94, equations 2.99 a-d).
Especially in frequency converter drives, the regulation of asynchronous motors is implemented using vector control. An advantage of vector control is that it allows the regulation of d.c. components. This obviates the need to provide each phase of a three-phase motor with a separate regulation arrangement. The book "Steuerverfahren fur Drehstrommaschinen", 1983, by Spath, describes the regulation of an asynchronous motor, especially a squirrel-cage motor, in a coordinate system tied to a space vector. On pages 6-33, this book presents a simple model of an asynchronous motor, in which the real axis is oriented in the same direction as the rotor flux vector. In addition, regulation systems are known which are based on the use of current regulators and which can be used in the rotor flux oriented regulation referred to. In such systems, the current has to be measured, and this makes them expensive and complex.